Most songwriters, I have spoken with, start withthe music for inspiration. This is not strictly necessary but it often helps set the mood of your song.

One of my favorite methods is bainstorming. I play with a new guitar riff and babble nonsense into a cheap recorder. This is constructive in two ways. First, it helps me remember the feel of the guitar and melodic arrangement I am exploring. Second, I can playback and listen for spontanious lyric ideas. Those perspectives form the two basic aspects of the song's foundation; Composition/Melody and Theme. The composition and melody form the essential feel for the song's meaning or purpose. As I play with the sound I search for pockets of meaning in my babbling.

I use a similar approach if I already have some music/ or lyric/theme to start with.

Once I have the music composition matched up to a verse/melody structure I begin going back and forth between playing/singing and writing more verses. I might include a bridge or save it for later. A bridge is a specially composed deviation from the repetitive verse structure that takes you to a new place or introduces a new perspective on the general theme.

Eventually, I try to get the song to a complete draft. By that I mean it has the basic arrangement and theme that I want. Then I continue to rewrite and rearrange lyrics and chords and verses to get closer to the meaning I am trying to convey. For example, I often find that the first verse I write belongs later in (or at the end of) the song because it convays the general idea rather than the specifics I wrote later.